The two types of cassettes referred to, aside from being usually distinguishable by their size, also differ in the way that the film is guided from a supply reel to a takeup reel. Thus, a silent-film cassette generally has only one location at which the film is accessible, i.e. the area which in operation confronts the image gate of the camera while a reciprocating transport claw engages in the perforations of the accessible film portion; in a sound-film cassette, on the other hand, there is also provided a cutout which the film reaches after passing the image gate and where that film is entrained by a rotating capstan and a coacting pressure roller past a recording head. Conventionally, cassettes of both types also have laterally accessible hubs by which their takeup reels can be driven through a catch within the camera operating concurrently with the transport claw. In order to prevent accidental reverse rotation of the takeup reel, especially upon removal of the cassette from the camera when such reverse rotation could expose a previously wound-up film portion to ambient light, the hub of that reel is normally provided with a rewind stop such as a pawl engaging a ratchet.
In certain instances, especially for the purpose of trick cinematography, it is desirable to rewind short stretches of film on the supply reel for a second exposure. Because of the aforementioned stop, such rewinding is possible with the usual cassette only if a reserve length of film is accumulated within the cassette downstream of the exposure gap. To this end it has already been proposed to slow down or possibly arrest the takeup reel during the first run of a double exposure under the control of a suitably programmed sequence switch; in a second stage of the program, while the slowdown or standstill of the takeup reel continues, the accumulated extra stretch is reverse-transported past the image gate (usually with obstruction of that gate by the camera shutter or by a diaphragm) whereas in the third stage the film is again advanced under normal wind-up conditions for another exposure. The various program steps, including the retardation or deactivation of the windup drive in the first stage, can be implemented by a set of cams controlled by a three-position selector.
The aforedescribed mode of operation is rather impractical in the case of a sound-film cassette since the reserve length of film tends to accumulate within the cutout provided for sound recording, thereby risking entanglement with the recording mechanism. It is therefore already been proposed to limit the use of such a double-exposure program to silent-film cassettes and to disable the corresponding sequence switch or selector when the camera is loaded with a sound-film cassette. Though this solution obviates the risk of damage to a sound film, it also deprives the user of the possibility of subjecting such a film to double exposure or other cinematographic tricks calling for a change in the transport direction.